Recombinant DNA technology is being used to refine our knowledge of the structural organization of the chicken genome to the point where well-studied structural genes that are responsive to hormones can be placed in context with respect to the structural organization of the genome as a whole. Our primary tools for studying DNA sequence organization in the chicken are cloned DNA fragments containing repetitive sequence elements, since these can be used to determine the structure of all the many different sequences in the genome to which the cloned sequence is related.